1. Field of the Invention
The present invention relates to an improved high-capacity compound mechanical change gear transmission structure, preferably for vehicular compound transmissions of the multiple substantially identical countershaft type. In particular, the present invention relates to an improved high-capacity compound mechanical transmission structure allowing, for a given transmission capacity and life, the provision of a lighter, axially shorter and/or less costly transmission as compared to comparable prior art high-capacity transmission structures.
More particularly, in one of the preferred embodiments, the reverse mainshaft and countershaft gears are of a reduced pitch diameter to provide ease of bearing assembly and assembly of the mainshaft into a front-opening, tub-type housing.
2. Description of the Prior Art
Compound change gear transmissions, usually vehicular transmissions for heavy duty vehicles, of the type having one or more auxiliary sections connected in series with a main transmission section are well known in the prior art. Briefly, by utilizing main and auxiliary transmission sections connected in series, assuming proper sizing of the ratio steps, the total of available transmission ratios is equal to the product of the main and auxiliary section ratios. By way of example, at least in theory, a compound change gear transmission comprising a 4-forward-speed main section connected in series with a 3-speed auxiliary section will provide twelve (4.times.3=12) available forward ratios.
Auxiliary transmission sections are of three general types: range type, splitter type or combined range/splitter type.
In compound transmissions having a range type auxiliary section, the auxiliary section ratio step or steps are greater than the total ratio coverage of the main transmission section ratios used in both ranges and the main section is shifted progressively through its ratios in each range. Examples of compound transmissions having range type auxiliary sections may be seen by reference to U.S. Pat. Nos. 3,105,395; 2,637,222; 2,637,221 and 4,807,493, the disclosures of which are incorporated herein by reference.
In compound transmissions having a splitter-type auxiliary section, the ratio steps of the splitter auxiliary section are less than the ratio steps of the main transmission section and each main section ratio is split, or subdivided, by the splitter section. Examples of compound change-gear transmissions having a splitter-type auxiliary sections may be seen by reference to U.S. Pat. Nos. 4,290,515; 3,799,002; 4,440,037 and 4,527,447, the disclosures of which are incorporated herein by reference.
In a combined range-and-splitter-type auxiliary section, or sections, both range and splitter type ratios are provided allowing the main section to be progressively shifted through its ratios in at least two ranges and also allowing one or more of the main section ratios to be split in at least one range.
One example of a compound transmission having a single combined range/splitter-type auxiliary section may be seen by reference to U.S. Pat. Nos. 3,283,613; 3,648,546, the disclosures of which are incorporated herein by reference. Another example is the "Ecosplit" model of transmission sold by Zahnradfabrik Friedrichshafen Aktiengeseushaft of Friedrichshafen, Federal Republic of Germany, which utilizes a separate splitter auxiliary section in front of, and a separate range auxiliary section behind, the main transmission section.
Another example of a compound vehicular transmission having a single combined 3-gear-layer, 4-speed combined splitter/range transmission may be seen by reference to U.S. Pat. Nos. 4,754,665 and 4,944,197, the disclosures of which are incorporated herein by reference.
It should be noted that the terms main and auxiliary sections are relative and that if the designations of the main and auxiliary sections are reversed, the type of auxiliary section (either range or splitter) will also be reversed. In other words, given what is conventionally considered a 4-speed main section with 2-speed range-type auxiliary section, if the normally designated auxiliary is considered the main section, the normally designated main section would be considered a 4-speed splitter-type auxiliary section therefor. By generally accepted transmission industry convention, and as used in this description of the invention, the main transmission section of a compound transmission is that section which contains the largest (or at least no less) number of forwards speed ratios, which allows selection of a neutral position, which contains the reverse ratio(s) and/or which is shifted (in manual or semiautomatic transmissions) by manipulation of a shift bar or shift rail or shift shaft/shift finger assembly as opposed to master/slave valve/cylinder arrangements or the like.
The prior art compound change gear transmissions, especially the prior art compound transmissions of the type having both range- and splitter-type auxiliary section gearing, such as the "Roadranger" and "Super 10" offered by Eaton Corporation and the "Ecosplit" offered by Zahnradfabrik Friedrichshafen Aktiengeseushaft, are well received and widely used in manually shifted, heavy-duty vehicles. However, these and other types of prior art compound change-gear transmissions are not totally satisfactory, as it is an ongoing objective, especially for vehicular transmissions, to provide transmissions of equal or improved capacities and reliability which use fewer parts and/or are axially shorter and/or lighter in weight as compared to prior art transmissions of comparable capacity and reliability.
U.S. Pat. Nos. 5,370,013 and 5,390,561, the disclosures of which are incorporated herein by reference, disclose transmissions of an improved structure which use fewer parts and/or are axially shorter and/or of a lighter weight as compared to comparable prior art compound transmissions of equivalent capacity and reliability. This was accomplished by providing a compound transmission wherein (i) the main and auxiliary section countershafts form a coaxial assembly of countershafts supported solely by bearings in the front and rear end walls of the transmission housing, and/or (ii) the mainshaft is supported, preferably in a radially floating manner, solely by the input and/or output shafts without intermediate bearings journalled in an intermediate housing wall. The above structure eliminated the requirement of all or a large portion of the intermediate housing wall(s) and the various shaft support bearings associated therewith which allowed the provision of compound change gear transmissions of relatively lighter weight, shorter axial length and/or fewer parts as compared to comparable prior art transmissions.
While the above-described improved transmission structure is an advancement in the art, to provide a compound transmission having a relatively equal or greater reliability and a relatively large capacity without providing some intermediate support for the countershaft assemblies requires an undesirable increase in transmission center distances (the distance between the axes of rotation of the input shaft/main shaft/output shaft and the countershaft assemblies) and/or diameters of the countershafts.